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The Dutch company that issued a rogue digital certificate for all Google Internet domains said today that its network had been hacked last month.

DigiNotar, a Dutch certificate authority (CA) that was acquired earlier this year by Chicago-based Vasco, said it was unaware of the breach for more than a week and had overlooked the in-the-wild Google certificate for over a month.

Multiple SSL (secure socket layer) certificates were stolen in the July hack, said DigiNotar.

"On July 19, 2011, DigiNotar detected an intrusion into its Certificate Authority (CA) infrastructure, which resulted in the fraudulent issuance of public key certificate requests for a number of domains, including Google.com," the company said in a statement on Tuesday.

"At that time, an external security audit concluded that all fraudulently issued certificates were revoked. Recently, it was discovered that at least one fraudulent certificate had not been revoked at the time," said DigiNotar, referring to the certificate valid for all Google properties.

The company did not provide any additional information about the intrusion, such as the origin of the attack or the number of certificates that had been issued to the intruders. DigiNotar has not replied to Computerworld's questions.

Today, however, a DigiNotar spokesman told Jeremy Kirk of the IDG News Service -- like Computerworld, part of IDG -- that "several dozen" certificates had been generated by the hackers.

DigiNotar's timeline shows that the company was unaware of the hack for over a week: The Google certificate was issued July 10, according to information posted to Pastebin.com last Saturday. DigiNotar did not revoke the Google certificate until Monday, Aug. 29.

Fraudulently-acquired certificates are dangerous because they can be used by criminals to conduct "man-in-the-middle" attacks targeting users of legitimate online services and websites. The fake Google certificate, for example, was used by attackers to target Iranian users of the Mountain View, Calif.-based search giant's services, Google said late Monday.

The DigiNotar hack was the second breach of a certificate-issuing firm since March, when Comodo admitted that hackers had used an account assigned to a company partner in southern Europe.to acquire nine certificates for some of the Web's biggest sites, including Google and Gmail, Microsoft, Skype and Yahoo.

Initially, Comodo argued that Iran's government may have been involved in the theft. Days later, however, a solo Iranian hacker claimed responsibility for stealing the SSL certificates.

On Monday, Google pointed a finger at Iran in the DigiNotar hack, saying that attacks using the ill-gotten certificate had targeted Iranian users.

Over the past 24 hours, Google, Microsoft and Mozilla have taken steps to block the rogue certificate.

Google will update Chrome "very soon," said a company spokesman today, to block all DigiNotar-issued certificates "while investigations continue."

According to Google, users of Chrome 13 and newer were protected against the rogue certificate because the browser has Google's legitimate certificates, and only Google's, hard-coded into it.

Yesterday, Mozilla promised that it would update both the desktop and Android editions of Firefox to revoke all DigiNotar certificates because "the extent of the mis-issuance is not clear."

Microsoft also weighed in with a security advisory of its own that announced it had nuked all DigiNotar certificates by adding the Dutch company's root to its list of banned certificates.

Windows Vista, Windows 7, Server 2008 and 2008 R2 users are now protected from attacks using any DigiNotar certificate, said Microsoft, but Windows XP and Server 2003 users are not. "Microsoft will release a future update to address this issue for all supported editions of Windows XP and Windows Server 2003," the advisory read.

XP still accounts for the largest chunk of Windows users, according to metrics vendor Net Applications, which measured the decade-old operating system's usage share at just under 50% last month.

Other browsers, such as Safari, will likely follow suit, said Chet Wisniewski, a security researcher with U.K.-based Sophos, as soon as Apple adds the DigiNotar root certificate to Mac OS X's blacklist.

"But Apple has a bad track record on this," said Wisniewski today, noting that Apple took three weeks last spring to update Safari after the Comodo hack disclosure.